1. Field of the Invention
This invention relates a duty solenoid valve control system for an automatic transmission.
2. Description of the Prior Art
As is well known, in an automatic transmission, the gear-shifting is effected by the alternate engagement and disengagement of friction elements such as a clutch, a brake and the like. The friction elements are actuated by hydraulic control valves which control the hydraulic pressure to be fed to the friction elements according to the pilot pressure given thereto.
Generally, the friction element is engaged when a hydraulic pressure higher than a predetermined value is fed thereto, and is disengaged when the hydraulic pressure fed thereto is lowered. It has been proposed as disclosed, for instance, in Japanese Patent Publication No. 62(1987)-18780 to carefully control the hydraulic pressure to be fed to the friction element by use of a duty solenoid valve which controls the pilot pressure given to the hydraulic control valve, thereby smoothly effecting the alternate engagement and disengagement of the friction element, and suppressing torque shock when the gears are shifted.
However, there is a problem in that the hydraulic pressure control properties change with the viscosity of the hydraulic oil which changes with temperature. The hydraulic pressure control properties are evaluated on the basis of the pressure regulation performance, the hydraulic pressure oscillation and the response performance.
In FIG. 6, line A1 is a pilot pressure change curve (a curve representing the change in the pilot pressure with duty cycle of the duty solenoid valve) for a hydraulic oil temperature of 80.degree. C. and a long driving period (frequency of 35 Hz), and curve V1 shows a pilot pressure oscillation waveform for a hydraulic oil temperature of 80.degree. C. and a long driving period (frequency of 35 Hz). Line A2 is a pilot pressure change curve for a hydraulic oil temperature of 80.degree. C. and a short driving period (frequency of 70 Hz), and curve V2 shows a pilot pressure oscillation waveform for a hydraulic oil temperature of 80.degree. C. and a short driving period (frequency of 70 Hz). Line A3 is a pilot pressure change curve for a hydraulic oil temperature of -10.degree. C. and a long driving period (frequency of 35 Hz), and curve V3 shows a pilot pressure oscillation waveform for a hydraulic oil temperature of -10.degree. C. and a long driving period (frequency of 35 Hz). Line A4 is a pilot pressure change curve for a hydraulic oil temperature of -10.degree. C. and a short driving period (frequency of 70 Hz), and curve V4 shows a pilot pressure oscillation waveform for a hydraulic oil temperature of -10.degree. C. and a short driving period (frequency of 70 Hz).
The duty solenoid valve regulates the pilot pressure by draining the hydraulic oil in response to receipt of an ON-signal, and the pilot pressure oscillation waveform curves V1 to V4 represent the change in the pilot pressure which occurs in response to the opening and closure of the duty solenoid valve.
A controller determines the value of a duty signal, which is expected to produce a target pressure, on the basis of the pilot pressure change curve selected according to the oil temperature and the driving frequency, and inputs the duty signal into the duty solenoid valve. The pressure regulation performance is evaluated on the basis of the deviation of the pilot pressure actually obtained in response the duty signal from the target pressure. The value of the duty signal determined by the controller is converted into an electric signal and then input into the duty solenoid valve. Since an electric signal is affected by voltage, the electric signal actually output can differ to some extent from the electric signal which the controller intends to output. That is, the driving duty cycle represented by the electric signal actually input into the duty solenoid valve differs from the driving duty cycle the controller has determined, which causes an error in the actual pilot pressure.
Further, as can be understood from the comparison of the pilot pressure change curves A1 to A4, the rate of reduction of the pilot pressure in the low duty cycle range (where the draining time is short) becomes very low when the oil temperature is low as compared with when the oil temperature is high. This is because, when the oil temperature is low, the viscosity of the hydraulic oil is high and the amount of oil drained in a unit time is small. This tendency is enhanced as the driving period is shortened and the draining time in one cycle becomes shorter. These facts cause the rate of change of the pilot pressure for a given change in the value of the duty signal to vary depending on the oil temperature and the driving period. Also the control width is caused to vary depending on the oil temperature and the driving period.
Accordingly, if it is assumed that the driving duty cycle which the controller determines corresponding to a target pilot pressure P1 is d1 and the driving duty cycle represented by the electric signal actually input into the duty solenoid valve is d2 with the pilot pressure corresponding to the driving duty cycle d2 represented by P2, the difference between the target pilot pressure P1 and the actual pilot pressure P2 corresponding to the driving duty cycle d2 is relatively small when the oil temperature is high and the driving period is short, while the difference between the target pilot pressure P1 and the actual pilot pressure P2 for the same difference between d1 and d2 is relatively large when the oil temperature is low and the driving period is short. That is, when the oil temperature is low and the driving period is short, the pressure regulation performance deteriorates. In other words, as the inclination of the pilot pressure change curve increases, the pressure regulation performance deteriorates more rapidly. Since the control width narrows as the inclination of the pilot pressure change curve increases, the pressure regulation performance deteriorates more rapidly, as the control width becomes narrower.
The pilot pressure oscillation is generated in response to the on and off states of the duty signal and is evaluated on the basis of the driving period and the amplitude of the oscillation. In the case of the pilot pressure oscillation waveforms V2 and V4 for the shorter driving period (70 Hz), the duty solenoid valve is turned on and off twice as often as in the case of the pilot pressure oscillation waveforms V1 and V3 for the longer driving period (35 Hz).
The pilot pressure oscillation generated while gear-shifting is being effected fluctuates the working hydraulic pressure to the brakes or the clutches and causes fluctuation in the torque transmitted by the brakes or the clutches. As a result, the pilot pressure oscillation is felt as a vibration of the vehicle body. Generally, vibration of the vehicle body is harder to feel as the frequency of the vibration increases. From this viewpoint, pilot pressure oscillation having a shorter period is preferable to that having a longer period.
The amplitude of the pilot pressure oscillation is affected by the oil temperature and the driving period. That is, so long as the driving period is the same, the amplitude of the pilot pressure oscillation is larger when the oil temperature is high than when the oil temperature is low as can be understood from the comparison of the pilot pressure oscillation waveforms V1 and V3 or V2 and V4. This is because the viscosity of the hydraulic oil is lowered as the oil temperature increases and the flow of the hydraulic oil during the on times increases, whereby the fluctuation in the pilot pressure increases. Even when the oil temperature is the same, the amplitude of the pilot pressure oscillation for a given duty cycle is larger when the driving period is longer than when the driving period is shorter as can be understood from the comparison of the pilot pressure oscillation waveforms V1 and V2 or V3 and V4. This is because when the driving period is long, the on time in each cycle is long, and accordingly, the amount of the hydraulic oil which flows in the on time in each cycle is larger.
The response performance is defined to be the rate of change of the pilot pressure in response to a change in the duty cycle, and tends to deteriorate as the oil temperature is lowered and the fluidity of the hydraulic oil drops. Further, the response performance improves as the driving period becomes shorter.
When a shorter driving period (e.g., 70 Hz) is adopted, the pilot pressure regulation performance is unsatisfactory when the oil temperature is low though the pilot pressure control properties are acceptable when the oil temperature is high. On the other hand, when a longer driving period (e.g., 35 Hz) is adopted, the pilot pressure oscillation and the response performance deteriorate when the oil temperature is high though the pilot pressure regulation performance is acceptable.